Improved superlattice film

Abstract

A superlattice film comprising a superlattice structure disposed between a first conductor and a second conductor, and the superlattice structure comprising a plurality of superimposed nanocrystal layers; wherein each of the layers comprises an array of nanocrystals having the same energy gap, and wherein the layers are ascending ordered by the energy gaps of nanocrystals from the first conductor toward the second conductor such that a maximum energy gap layer is adjacent to the first conductor and a minimum energy gap layer is adjacent to the second conductor; the superlattice film further includes at least one of an electron blocking layer between the maximum energy gap layer and the first conductor, and an electron transport layer between the minimum energy gap layer and the second conductor.

Description

Technical Field

[0001] The present invention relates to a superlattice film, particularly but not exclusively for use in photovoltaic devices such as thin film solar cells, and modular devices comprising a plurality of superlattice films. Background Art

[0002] As we all know, solar cells, or photovoltaic cells, are devices that convert light energy directly into electrical energy through the photovoltaic effect.

[0003] The most relevant property of a solar cell is its efficiency.

[0004] First generation solar cells - also known as conventional, traditional or water-based cells - are typically made of crystalline silicon, and more specifically, include materials such as multicrystalline silicon and monocrystalline silicon. Individual conventional solar cells are typically combined to form modules, also known as solar panels.

[0005] Over time, the second generation of solar cells, often called thin film solar cells, has been developed.

[0006] Known thin film solar cells are typically made by depositing one or more thin layers or thin films (TF) of photovoltaic materials on a substrate such as glass, plastic or metal. Thin films typically include materials such as cadmium telluride (CdTe), copper indium gallium diselenide (CIGS) and amorphous thin film silicon (a-Si, TF-Si).

[0007] The film thickness varies from a few nanometers (nm) to tens of micrometers (μm), so thin-film solar cells are much thinner than conventional silicon-based solar cells. This makes thin-film solar cells more flexible and lightweight, and therefore more versatile than crystalline silicon solar cells. In addition, these known thin-film solar cells are cheaper than conventional crystalline silicon solar cells.

[0008] For example, thin-film solar cells are now commonly used to build integrated photovoltaics and as semi-transparent photovoltaic glazing materials that can be laminated onto windows.

[0009] However, under existing technology, thin-film solar cells are less efficient than traditional crystalline silicon solar cells.

[0010] In practice, known thin-film solar cells have a maximum efficiency of about 10%.

[0011] Recently, new materials have been investigated that can be used as absorber photovoltaic materials in thin film solar cells, among which are superlattice structures.

[0012] As is known, the mentioned superlattice is a periodic structure consisting of an array of nanocrystals (also called "quantum dots" or "quantum wires"), which are semiconductor particles of a few nanometers in size. More precisely, such nanocrystals have a size smaller than the Bohr radius of their constituent substances, which gives them special optical and electronic properties due to quantum effects.

[0013] Typically, photovoltaic superlattices are isotropic structures in which the nanocrystals are actually shuffled.

[0014] Although these photovoltaic superlattices are theoretically very promising, they have so far not achieved sufficient efficiency to be useful in commercial applications.

[0015] In fact, currently used thin-film solar cells with superlattices as the absorbing photovoltaic material have efficiencies of about 8%-12%.

[0016] WO 2021 / 070169 discloses an improved superlattice structure for a thin-film solar cell, which comprises a plurality of superimposed nanocrystal layers and is configured to generate an electron flow between the layers when it is irradiated with radiation. Each layer comprises an array of nanocrystals having substantially the same size and shape, and the nanocrystals of each of the layers have different sizes and / or different shapes relative to the nanocrystals of the other layers. In fact, in WO 2021 / 070169, the layers are arranged in such an order that the superlattice structure is anisotropic along the lateral direction of the required conductivity. Due to the influence of this feature, in the superlattice structure of WO 2021 / 070169, the electron e-flow has a preferred direction (i.e., the lateral direction).

[0017] The superlattice structure disclosed by WO 2021 / 070169 substantially improves the efficiency of thin-film solar cells, however further improvements are still possible and desirable, particularly in terms of efficiency and versatility. Summary of the invention

[0018] The object of the present invention is to solve the above technical problems, eliminate the disadvantages of the background art and overcome its limitations, and provide a superlattice film with improved efficiency relative to the prior art.

[0019] Within the scope of this aim, an object of the present invention is to provide a superlattice film which is easy to produce and has competitive costs.

[0020] Furthermore, an object of the present invention is to provide a superlattice film with high versatility.

[0021] Another object of the invention is also to provide alternatives to the known solutions.

[0022] This object, and these and other goals as will become more apparent hereinafter, are achieved by a superlattice film comprising a superlattice structure disposed between a first conductor and a second conductor, and comprising a plurality of superimposed nanocrystal layers;

[0023] wherein each of the layers comprises an array of nanocrystals having the same energy gap, and wherein the layers are ordered in ascending order of energy gap of the nanocrystals from the first conductor toward the second conductor such that a maximum energy gap layer is adjacent to the first conductor and a minimum energy gap layer is adjacent to the second conductor;

[0024] The superlattice film further comprises at least one of the following:

[0025] - an electron blocking layer between the maximum bandgap layer and the first conductor, and

[0026] - An electron transport layer between the minimum bandgap layer and the second conductor.

[0027] This object and these aims are also achieved by a modular device according to claim 11 . BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The foregoing and further features and advantages of the present invention will become more apparent from the following description of a preferred but non-exclusive embodiment of a superlattice film according to the present invention, and a modular device comprising the same, which is illustrated by way of non-limiting example in the accompanying drawings, in which:

[0029] Figure 1 is a schematic diagram of a superlattice film according to the present invention;

[0030] Figure 2 is a schematic diagram of a superlattice structure included in a superlattice film according to the present invention;

[0031] Figure 3 is a schematic diagram of an alternative superlattice structure;

[0032] Figure 4 is a schematic diagram of a superlattice film according to the present invention used in a photovoltaic device;

[0033] Figure 5 is a schematic diagram of a modular device comprising two superlattice films according to the present invention.

[0034] It should be noted that the above drawings must be regarded as schematic diagrams as they do not reflect an exact scale in order to better illustrate the underlying structure of the invention. DETAILED DESCRIPTION

[0035] Referring to the cited drawings, a superlattice film, generally indicated by reference numeral 1 , includes a superlattice structure 10 disposed between a first conductor 91 and a second conductor 92 (ie, conductive elements).

[0036] The conductors 91, 92 are preferably conductive layers.

[0037] The superlattice structure 10, 100 comprises a plurality of superimposed nanocrystals 41-50 (or Figure 3 21-25 and 31-34) of layers 4A-4L (or Figure 3 2A-2E and 3A-3D in Figure 1).

[0038] Each layer 4A-4L, 2A-2E, 3A-3D contains nanocrystal arrays 41-50, 21-25, 31-34 with the same energy gap (it is known that the energy gap in a nanocrystal is the energy difference between the bottom of the conduction band and the top of the valence band of an electron). In fact, in order to have the same energy gap, all nanocrystals in the same layer 4A-4L, 2A-2E, 3A-3D have the same size and shape. It should be noted that in this specification and the attached claims, the term "shape" should be understood as referring only to the geometric shape (i.e., geometric structure) of the nanocrystal, without considering its size.

[0039] In the superlattice structures 10, 100, the layers 2A-2L, 3A-3L, 4A-4L are arranged in ascending order from the first conductor 91 toward the second conductor 92 according to the energy gaps of the nanocrystals 41-50, 21-25, 31-34. In other words, the layers 2A-2L, 3A-3L, 4A-4L are arranged in such an order that the energy gaps of the nanocrystals 41-50, 21-25, 31-34 decrease from the first conductor 91 to the second conductor 92.

[0040] Typically, all layers 4A-4L, 2A-2E, 3A-3E are then ordered in ascending order (along the transverse direction Y of the desired conductivity) from the first conductor 91 to the second conductor 92 according to the size of the nanocrystals 21 -25, 31 -34.

[0041] In fact, the energy gap in a nanocrystal is inversely proportional to the size of the nanocrystal.

[0042] Therefore, if Figure 1 As shown, the maximum bandgap layer 4L, 2E (ie, the layer containing nanocrystals 50 with the largest bandgap) is adjacent to the first conductor 91 , and the minimum bandgap layer 4A, 2A (ie, the layer containing nanocrystals with the smallest bandgap) is adjacent to the second conductor 92 .

[0043] In this way, in the superlattice structure 10 , 100 , electrons e − are induced to flow along the lateral direction Y from the maximum-gap layer 4L toward the minimum-gap layer 4A, but not vice versa.

[0044] exist Figure 1 and Figure 2 In FIG. 4 , for simplicity only, nanocrystals are depicted as spherical to represent any possible shape: nanocrystals 41 - 50 may have any suitable shape, such as hexadecahedrons, pentahedrons, octahedrons, cuboctahedrons, hexagons, and the like.

[0045] In a preferred embodiment, all nanocrystals 41 - 50 of the same layer 4A- 4L have the same size, so each layer 4A- 4L differs from the other layers only by the size of the nanocrystals 41 - 50 .

[0046] However, in some alternative embodiments, such as Figure 3 As depicted in FIG, superlattice structure 100 includes a first type of layers 2A-2E including nanocrystals having a first shape, and a second type of layers 3A-3D including nanocrystals having a second shape different from the first shape; in this case, the first type of layers 2A-2E alternate with the second type of layers 3A-3D.

[0047] As for the composition of nanocrystals 21-25, 31-34, they are made of semiconductor materials such as: CdS, CdSe, CdTe, InP, InAs, ZnS, ZnSe, HgTe, GaN, GaP, GaAs, GaSb, InSb, Si, Ge, AlAs, AlSb, PbSe, PbS, PbTe, InGaAs, InGaN, AlInGaP.

[0048] In a preferred embodiment, the nanocrystals 21 - 25 , 31 - 34 are made of one or more of the following materials: PbSe, PbS, PbTe, CdS, CdSe, CdTe.

[0049] Preferably, all nanocrystals 21 - 25 , 31 - 34 are made of the same material.

[0050] In fact, the superlattice structure 10, 100 can be any superlattice structure described in WO 2021 / 070169.

[0051] Advantageously, nanocrystals 41 - 50 are fixed at predetermined positions within layers 4A- 4L in such a way that they have both energetic and mechanical alignment.

[0052] In particular, it should be noted that within the superstructure 10, 100, the nanocrystals are fixed in predetermined positions in such a way that they have an energy alignment. In fact, the energy gaps of the nanocrystals are aligned so that electrons e- (excited by radiation S absorption) are allowed to traverse the entire superstructure 10, 100.

[0053] It should also be noted that within the superlattice structure 10, 100, the nanocrystals 41-50, 21-25, 31-34 are fixed in predetermined positions in such a way that they have a shape orientation alignment.

[0054] In more detail, the shape and orientation of the nanocrystals 41 - 50 , 21 - 25 , 31 - 34 are provided such that the nanocrystals have not only energetic alignment but also mechanical alignment.

[0055] Finally, in a preferred embodiment, the nanocrystals 41-50, 21-25, 31-34 are fixed at predetermined positions within the layers 4A-4L, 2A-2E, 3A-3E in such a way that they have both energetic and mechanical alignment.

[0056] Advantageously, the spacing and connections between the nanocrystals 21-25, 31-34 are controlled by ligand molecules attached to the nanocrystals 21-25, 31-34.

[0057] Due to this synergistic combination of energy and mechanical alignment, within the superlattice structure 10, 100, the probability that an electron e-excited in a nanocrystal 41-50, 21-25, 31-34 due to the absorption of a photon will "jump" (move) to an adjacent nanocrystal 41-50, 21-25, 31-34 in the lateral direction Y (towards contact with the second conductor 92) is very high, and the probability that such an electron e-"jumps" (moves) to other adjacent nanocrystals 41-50, 21-25, 31-34 in other directions is very low.

[0058] According to the present invention, the superlattice film 1 further comprises at least one of the following:

[0059] - an electron blocking layer (EBL) 81 (also called a hole transport layer (HTL)) interposed between the maximum bandgap layer 4L and the first conductor 91, and

[0060] An electron transport layer (ETL) 82 (also called a hole blocking layer (HBL)) interposed between the minimum bandgap layer 4A and the second conductor 92 .

[0061] Preferably, as in the illustrated embodiment, the superlattice film 1 includes both the electron transport layer 82 and the electron blocking layer 81 .

[0062] As is known, electron transport layer 82 is a layer having physical properties (such as charge mobility, energy level alignment, defect states, morphology, and related interface properties) that make it useful in extracting and transporting excited electron carriers and acting as a hole blocking layer by suppressing charge recombination.

[0063] For example, the electron transport layer 82 may be made of one of the following materials: SnO2, CdSe, WO3, ZnSnO4, ZnO, PbI2, TiO2, SrTiO3, CH3NH3BbI3, ZnO2, SnO.

[0064] As is known, the electron blocking layer 81 has substantially the opposite effect to that of the electron transporting layer, which reduces the leakage of electrons to the first conductor 91 .

[0065] For example, the electron blocking layer 81 can be made of one of the following materials: Spiro-OMeTAD, PEDOT:PSS, PTAA (poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine]), P3HT, DM, TAT-tBuSty, X26, X36, FDT, SCZF-5, TTE, PTEG, cuprous oxide (Cu2O), cupric oxide (CuO), cuprous thiocyanate (CuSCN), cuprous iodide (CuI), nickel oxide (NiO x ), MoS2, WS2, SANs, Cu(Tu)I, MnS, CuS, copper indium gallium disulfide (CIGS) nanocrystals, such as Cu(In 0.75 Ga 0.25 )S2 and Cu(In 0.5 Ga 0.5 )S2.

[0066] The presence of the electron transport layer 82 and / or the electron blocking layer 81 extends the work function between the conductors 91, 92 and the nanocrystals, thus increasing the efficiency of the superlattice film 1, particularly when used as a photovoltaic device.

[0067] To allow absorption by the superlattice structure 10, at least one conductor, ie the first conductor 91, is at least partially transparent to light, preferably transparent to visible light, and even more preferably completely transparent to light.

[0068] Figure 4 A superlattice film 1 used as a photovoltaic device (as a solar cell) is shown: a first conductor 91 and a second conductor 92 are connected by a circuit so that due to solar radiation S, electrons e- flow from the first conductor 91 to the second conductor 92 along a lateral direction Y (and therefore a current c flows in the opposite direction in the circuit).

[0069] As described above, two or more superlattice films 10 may be combined to form a modular device 110, such as Figure 5 A modular device as described.

[0070] In more detail, the modular device 110 includes at least two superlattice films 1, 1', a first superlattice film 1 and a second superlattice film 1', which are stacked together along the lateral direction Y so that the minimum energy gap layer 4A of the first superlattice film 1 faces the minimum energy gap layer 4A' of the second superlattice film 1'.

[0071] The conductive layer 92 is interposed between the minimum bandgap layer 4A of the first superlattice film 1 and the minimum bandgap layer 4A′ of the second superlattice film 1 ′, so that this single conductive layer 92 constitutes the second conductor 92 of both the first superlattice film 1 and the second superlattice film 1 ′.

[0072] In addition to the first superlattice film 1 and the second superlattice film 1', the modular device 110 may also include other superlattice films stacked in the same manner: for example, at least a third superlattice film (not shown), which is stacked on the second superlattice film 1' so that the maximum energy gap layer of the third superlattice film faces the maximum energy gap layer 4L' of the second superlattice film 1'; in this case, a single conductive layer constitutes the first conductor 91' of both the second superlattice film 1' and the third superlattice film.

[0073] The fourth superlattice film may be stacked on the third superlattice film such that the minimum energy gap layer of the fourth superlattice film faces the minimum energy gap layer of the third superlattice film 1 ′ and so on.

[0074] In other words, the modular device 110 comprises a series of superlattice structures 10 , 100 alternating with conductive layers 91 , 92 .

[0075] In the operating configuration of the modular device 110, when the modular device 110 is illuminated by light, the conductors 911, 92, 91' are electrically connected to generate an electric current in the circuit.

[0076] It should be noted that within the superlattice structure 10, a gradient is provided that creates a "superconductor" in the conduction direction (lateral direction Y), so the superlattice film 1 (and modular device 110) can be used in any application requiring such behavior (e.g. for the manufacture of supercapacitors).

[0077] According to an alternative and simpler solution, the superlattice structure 10 alone may be used for applications requiring "superconductor" behavior, such as supercapacitors.

[0078] The operation of the superlattice film is clear and obvious from the above description.

[0079] In practice, it has been found that the superlattice film according to the invention achieves the intended aim and objects since it allows an increase in efficiency with respect to the prior art.

[0080] Another advantage of the superlattice film according to the invention is that it allows providing a thin film solar cell with high versatility.

[0081] Another advantage of the superlattice film according to the present invention is that it is highly reliable, relatively easy to manufacture and cost competitive.

[0082] Furthermore, the superlattice film according to the invention provides an alternative to the known solutions.

[0083] The invention thus conceived is susceptible of numerous modifications and variations, all of which are within the scope of the inventive concept; moreover, all the details may be replaced by other technically equivalent elements.

[0084] In practice, the materials used, as well as the dimensions, may be chosen at will according to requirements and the state of the art.

[0085] The scope of the invention is thus indicated by the appended claims rather than the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced.

Claims

1. A superlattice film, the superlattice film comprising a superlattice structure arranged between a first conductor and a second conductor, and the superlattice structure comprising a plurality of superimposed nanocrystal layers; wherein each of the layers comprises an array of nanocrystals having the same energy gap, and wherein the layers are ordered in ascending order of energy gap of the nanocrystals from the first conductor toward the second conductor such that a largest energy gap layer is adjacent to the first conductor and a smallest energy gap layer is adjacent to the second conductor; The superlattice film further comprises at least one of the following: - an electron blocking layer between the maximum bandgap layer and the first conductor, and - an electron transport layer between the minimum bandgap layer and the second conductor. 2 . The superlattice film according to claim 1 , comprising both the electron transport layer and the electron blocking layer.

3. The photovoltaic device according to claim 1 or 2, wherein the electron transport layer is made of one of the following materials: SnO, SnO2, CdSe, WO3, ZnSnO4, ZnO, ZnO2, PbI2, TiO2, SrTiO3, CH3NH3BbI3.

4. The superlattice film according to any one of the preceding claims, wherein the electron blocking layer is made of one of the following materials: Spiro-OMeTAD, PEDOT:PSS, PTAA, P3HT, DM, TAT-tBuSty, FDT, SCZF-5, TTE, PTEG, Cu2O, CuO, CuSCN, CuI, NiO x , MoS2, WS2, SANs, Cu(Tu)I, MnS, CuS, CIGS nanocrystals.

5. A superlattice film according to any one of the preceding claims, wherein the first conductor is at least partially transparent to light, preferably to visible light.

6. The superlattice film of any of the preceding claims, wherein the layers are ordered in ascending order from the first conductor to the second conductor according to the size of the nanocrystals.

7. The superlattice film of claim 1, wherein the layer comprises: - a first type of layer comprising nanocrystals having a first shape, - a second type of layer comprising nanocrystals having a second shape different from the first shape; The layers of the first type alternate with the layers of the second type.

8. The superlattice film of claim 1, wherein the nanocrystals are fixed at predetermined locations within the layer in such a way that they have both energetic and mechanical alignment.

9. The superlattice film of claim 1, wherein the nanocrystals are made of one or more of the following materials: CdS, CdSe, CdTe, InP, InAs, ZnS, ZnSe, HgTe, GaN, GaP, GaAs, GaSb, InSb, Si, Ge, AlAs, AlSb, PbSe, PbS, PbTe, InGaAs, InGaN, AlInGaP.

10. The superlattice film of claim 1, wherein the nanocrystals are made of one or more of the following materials: PbSe, PbS, PbTe, CdS, CdSe, CdTe.

11. A modular device comprising at least two superlattice films according to any one of the preceding claims, a first superlattice film and a second superlattice film, which are stacked so that the minimum energy gap layer of the first superlattice film faces the minimum energy gap layer of the second superlattice film; A single conductor layer constitutes the second conductor of both the first superlattice film and the second superlattice film.

12. A modular device according to claim 11, wherein the modular device also includes at least a third superlattice film, which is stacked on the second superlattice film so that the maximum energy gap layer of the third superlattice film faces the maximum energy gap layer of the second superlattice film; wherein a single conductive layer constitutes the first conductor of both the second superlattice film and the third superlattice film.

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